Secondary batteries are widely known as secondary batteries for vehicle driving. Such a secondary battery includes following components: a positive electrode sheet and a negative electrode sheet, both forming part of an electricity-generating element group, a separator that separates the positive electrode sheet and the negative electrode sheet from each other, and an electrolyte. These components are housed together in an enclosed battery container made of a metal or resin. The secondary battery also includes external terminals fixed to the battery container and electrically continuous with the electrodes that form part of the electricity-generating element group. Lithium-ion secondary batteries are typical secondary batteries of this kind.
A large number of conventional lithium-ion secondary batteries have had a cylindrical shape in appearance. On the other hand, developing unit-cell batteries of a rectangular shape has recently come to be studied. These unit-cell batteries are intended to improve packaging density (volumetric capacity density) by combining tens of, or more than a hundred, unit cells and constructing a battery pack or battery module for vehicle-mounting specifications in response to the need for higher power output and capacity.
For example, JP-2000-150306-A discloses a battery or capacitor. In such a disclosure, a case cover is applied over an opening in a rectangularly shaped and aluminum-made case body accommodating an electrode structure (hereinafter, referred to as the electricity-generating element group), and then the connection is caulked or staked via a gasket, to hermetically seal the case. The case cover, made from a phenol-based resin, is formed with a terminal-mounting hole for fixing a current-collector terminal member at both ends of the cover, and an electrolyte-filling hole for filling the battery with an electrolyte at a central portion of the cover. The electricity-generating element group includes a current-collector foil laminate that is not coated with an active material mixture at an end portion of the laminate and has only current-collector foil sections formed into lamination (this foil laminate is hereinafter referred to as the non-coated section), and the element group is electrically continuous with a region inclusive of external terminals as well as clamping-type current-collector members for clamping the non-coated section. The non-coated section is resistance-welded at three places on each current-collector foil in such a form as to bias the foil in the direction that it is clamped (refer to JP-2000-150306-A, FIG. 4, reference number 14).
In another example of a rectangular battery, as disclosed in JP-2007-226989-A, an electrode group (hereinafter, referred to as the electricity-generating element group) with exposed sections of cores formed at both ends (these exposed sections are hereinafter referred to as the non-coated sections) is housed in a metallic outer casing of a rectangularly parallelepipedic shape. The electricity-generating element group is formed by interposing, between a positive electrode sheet and a negative electrode sheet, a band-shaped separator formed from a microporous membrane, and after winding the electrode sheets and the separator into spiral form, taping this spiral body at its outermost region for flattening. A positive-electrode non-coated section is formed at one end of the electricity-generating element group, and a negative-electrode non-coated section at the other end. The positive-electrode non-coated section and the negative-electrode non-coated section are welded in a bundled condition onto a positive current collector and a negative current collector, respectively (these current collectors are hereinafter referred to as the connecting sheets). In other words, the non-coated sections and the connecting sheets are laser-welded over a predetermined length by laser beam irradiation with the main body of one connecting sheet pressed against each non-coated section.